Optical fiber has now largely replaced copper conductors in long line telecommunications cable and is widely used for data transmission as well. Increased use of fiber optics in local loop telephone and cable TV service is expected, as local fiber networks are established to deliver ever greater volumes of information, in the form of data, audio, and video signals to residential and commercial users. In addition, use of optical fibers in the home and in businesses for internal data, voice, and video communications has begun and is expected to increase.
One of the principal drawbacks to the use of optical fibers is the difficulty in achieving an end-to-end splice with acceptable light transmission loss. For a good connection, the two fibers must be aligned very precisely. At present, this requires a high level of skill by the installer, as well as more time and more expensive tools relative to installations employing metallic conductors. Moreover, this problem, though important in long line transmission fibers, is exacerbated when the fiber is used in local applications, where the number of splices per unit length of fiber installed is greatly increased.
Optical fiber ribbons provide a modular design which simplifies the construction, installation, and maintenance of optical fiber cable by eliminating the need to handle individual fibers. An optical fiber ribbon is constructed of a plurality of optical waveguides, each of which is typically coated with one or more polymeric coatings which serve to protect and cushion the waveguide. The plurality of coated waveguides, each of which is frequently referred to as an optical fiber, is held in a coplanar arrangement by a ribbon matrix material which bonds the individual optical fibers to each other or surrounds the plurality of optical fiberb-in-a common outer jacket or sheathing.
Use of optical fiber ribbons promises to reduce the labor and cost involved in splicing individual optical fibers, because the optical fibers in the ribbon can be spliced by connecting the much larger ribbon, provided that the positions of the optical fibers therein can be precisely fixed and maintained. In one method commonly used to splice ribbons, known as mass fusion splicing, the first step involves the complete removal of all protective polymer coatings and the ribbon matrix material. The process relies upon a V-block to align the individual fibers. The V-block controls angular alignment particularly well so long as the optical waveguide is free of any protrusions, such as nonuniform primary coating material residue, in the region where the optical waveguide contacts the V-block. In addition, the V-block permits precise alignment of the two optical waveguide ends so long as the residual primary coating material on the two ends has the same thickness. Consequently, alignment of the two optical waveguides and the success of the mass fusion splice depend on the removal of the protective coatings. Indeed, if the coating materials cannot be cleanly and easily stripped, splicing operations using the V-block and other similar devices will be seriously hampered.
The need to remove completely the primary coating from the optical waveguide must be balanced with the coating's role of protecting the fiber waveguide from mechanical stresses, moisture infiltration, to which the silica material from which the optical waveguide is typically constructed is particularly susceptible, and other environmental hazards. Protecting the optical waveguide from these hazards is likely to become of increased concern, especially as the use of optical fibers in local data, audio, and video signal transmission grows. In contrast to the comparatively hermetic conditions in long distance cables, where fiber exposure points are far fewer and more sheltered, local optical fibers, having a vastly larger number of splices, are more prone to attack from a variety of environmental hazards. For example, optical fiber connections are commonly made in neighborhood pedestals, which are frequently unsealed, giving insects and animals access to the optical fiber and exposing the optical fiber to moisture and water. Moreover, a substantial percentage of fiber optic cables will find installation in existing pipe chases, including pipe chases containing steam lines, where there are risks to the coatings from thermal damage, alone and in combination with high humidity, to say nothing of direct steam impingement. The ability of the coatings to protect the optical waveguide from mechanical stresses and moisture has been correlated with the strength of the wet adhesive forces between the primary coating and the optical waveguide.
The dual requirements of strong bonding of the primary coating to the waveguide and ease and uniform strippability have presented a difficult challenge in primary coating formulation. The present invention is directed to meeting these dual requirements of adhesion and strippability.